Withdrawal strength of nailed joints with decay degradation of wood and nail corrosion

Nailed timber joints are widely used in timber structures, and their deterioration may cause significant damage. We investigated the withdrawal strength of joints using steel wire nails in specimens exposed to a brown-rot fungus. We also examined the effects of nail corrosion on withdrawal strength, because high humidity conditions accelerate not only wood decay but also the corrosion of nails. We found that nail corrosion increased the withdrawal strength. The ratios of withdrawal strength of nailed joints with rusted nails to that of joints with a minimally rusted nails were 1.47 and 1.56 in joints nailed in radial and tangential directions to annual rings, respectively. Withdrawal strength, excluding the effects of nail corrosion, had a negative correlation with mass loss and Pilodyn-pin-penetration-depth-ratio. We estimated the withdrawal strength of the nailed joint with decayed wood and rusted nails by multiplying the values from the empirical formula (obtained from mass loss and Pilodyn-pin-penetration-depth-ratio) by 1.47 and 1.56 for joints nailed in radial and tangential direction to annual rings, respectively.     

Thermal degradation modeling for single-shear nailed connections

The load-carrying capacity of a timber structure is highly dependent on the strength of its connections. Very limited information exists on the thermal degradation, or the properties at elevated temperatures, of such connections. This insufficient information is one of the major impediments for modeling elevated temperature performance for wood-frame structures. The objective of this study was to characterize thermal degradation of yield load capacity of single-shear nailed connections between wood and oriented strand board (OSB) as a function of time and temperature. The mechanism of degradation was explained using first order kinetics. Using the principles of time–temperature superposition (TTS), predictive models for connection capacity degradation were developed. A total of 450 wood-to-OSB connections were tested laterally after exposure to nine different elevated temperature regimes. The degradation of yield load strength over time occurred at a constant linear rate. Temperature dependence of this rate was modeled using the Arrhenius activation energy theory. Using Arrhenius activation energy theory and principles of TTS, a master curve was developed to predict the performance of connections after exposure to a temperature of 150°C. The predictions matched well with independent experimental observations. The master curve developed using TTS provides predictive estimates of residual capacity in a connection as well as its failure times.     

Yield load prediction of nailed timber joints using nail diameter and timber specific gravity

Prediction of yield load has nowadays been accepted as the basis for the limit state method of the design. Currently, there are many methods to predict the yield load of timber joints. These methods, i.e Foschi, Johansen, Smith and US 5% nail diameter offset, are reviewed in-depth in this paper. In this study, the authors presented a new approach which, hopefully, is simpler than the existing methods. The paper outlines the works carried out to arrive at the formula. About 300 Malaysian timber nailed joints were fabricated and tested to obtain the relationship between yield load and maximum load and to propose a new method to predict the yield load of a nailed joint. There seems to be a direct relationship between the yield load and the maximum load. From this relationship, a new formula, which depends only on nail diameter and specific gravity, was established.     

Estimation of shear strength of dowel-type timber connections with multiple slotted-in steel plates by European yield theory

The shear strength of dowel-type timber connections with multiple slotted-in steel plates was estimated based on European yield theory. The values calculated based on the yield theory were compared with experimentally obtained results. An experiment was performed on dowel-type timber connections with two and three slottedin steel plates under lateral loads parallel to the grain. The yield mode of the dowel-type connection assumed in this study corresponds approximately to the failure mode of the connection obtained from the experiment. The shear strength of the dowel-type connections calculated based on the yield theory showed good agreement with the results for shear strength obtained in the experiment. The yield theory was useful for estimating the shear strength of the doweltype connection with multiple slotted-in steel plates. The shear strength of the dowel-type connection was greatly affected by the spacing of the steel plates, the number of steel plates, and the timber thickness. The values of these parameters that showed the proper shear strength of the dowel-type connection could be estimated based on the yield theory. Part of this paper was presented at the 9th Timber Engineering Forum of the Japan Timber Engineering Society, December 2005     

Lateral deflections of timber beam-columns during drying

Summary A first approximation model for the mechano-sorptive deformation of wood is used to predict the deformation behaviour of beam-columns. An experimental check showed that the theory provides a good estimate of the lateral deflections that occur during the first drying cycle of initially green beam-columns.     

Withdrawal capacity of washers in bolted timber connections

Abstract

The European yield model (EYM) has been accepted to determine the load-carrying capacity of structural timber connections. However, experiments of bolted connections are still not in agreement with the EYM unless the additional bearing capacity offered by the washers is taken into account. This bearing capacity is depending on the compressive strength perpendicular to grain of the structural timber. Tests results carried out with M16 and M24 washers on solid and glued laminated timber are used to verify the reliability of three strength capacity predicting models, one of which is an analytical model while the other two are empirical. It was concluded that the analytical model is the superior one. This model should be incorporated in all new structural timber design code revisions.     

Modelling the long-term strength of timber columns

Summary This paper studies the influence of the used creep function, and the duration of the experimental sustained loading from which the creep function was derived, in modelling the long-term buckling strength of timber columns. The sensitivity of this buckling strength to creep and initial deflection is numerically studied using an energy approach that takes into account the initial deflection. Various creep functions — power laws as well as exponential laws — derived from bending experiments of different durations are used for the numerical simulation. The crucial need for experimental results issued from very long load period experiments is identified. Such experiments may lead to a more credible creep analysis of timber structures.Symbols i radius of giration - s stress level - E_L effective MOE - E₀ instantaneous MOE - I inertia moment - K,K geometrical matrices - L length of beam or column - L_f effective length - N_cr axial force, buckling load - U elastic strain energy - W external forces work - t time variable - u moisture content (m.c.) - v total deflection - v_c, v_e, v₀ creep, instantaneous elastic and initial deflections respectively Greek Letters (t) time function - , _e, ₀ total, instantaneous and initial strains respectively - twist rotation - (t) creep coefficient or fractional creep or relative creep - (x) shape function - load factor - longitudinal shortening - total potential energy - creep factor - relaxation time     

Effect of decay on shear performance of dowel-type timber joints

Monotonic and reversed cyclic loading tests were conducted on dowel-type timber joints with varying degrees of wood decay due to Fomitopsis palustris (Berk. et Curt.), a brown rot fungus, and the effect of decay on various shear performances of dowel-type joints was investigated. For joints affected by the brown rot fungus, the initial stiffness, yield load, and maximum load of dowel-type joints were significantly decreased, even with a small mass loss of wood. The reductions in shear performance were the largest for initial stiffness, followed by yield load and maximum load, in that order. For a 1% reduction of the yield load, initial stiffness and maximum load showed reductions of 1.15% and 0.77%, respectively. When dowel-type joints that had been exposed to the brown rot fungus were subjected to reversed cyclic loading, the gap between the dowel and the lead hole of the wood was increased and equivalent viscous damping was decreased. These results indicate that decay around the dowel lead hole especially affects the load-displacement behavior at small displacement level, and dowel-type joints under cyclic loading have very low resistance to forces acting on the wooden structure. Part of this report was presented at the 5th Symposium on Timber Bridges of the Japan Society of Civil Engineers, Tokyo, July 2006; the 56th Annual Meeting of the Japan Wood Research Society, Akita, August 2006; and the Annual Meeting of the Architectural Institute of Japan, Fukuoka, August 2007     

Probabilistic strength prediction of adhesively bonded timber joints

The strength prediction of adhesively bonded timber joints is difficult due to the anisotropic and brittle nature of the adherends, the complex stress distribution as well as the uncertainties regarding the associated material resistance. This paper describes a probabilistic method for the strength prediction of balanced double lap timber joints. The method considers the statistical variation and the size effect in the strength of timber using a Weibull statistical function. The design method presents an explanation for the increased resistance of local zones subjected to high stress peaks as it takes into account not only the magnitude of the stress distributions, but also the volume over which they act. The predicted joint strengths are slightly underestimated compared with the experimental results due to inaccurate upper tail modelling of the material strength by the Weibull statistical distribution. The probabilistic method provides reasonable results for brittle joint failure and has immediate application in the design of adhesively bonded timber joints.     

Fracture mechanics analysis of row shear failure in dowelled timber connections

A quasi-nonlinear fracture mechanics model is presented for the analysis of row shear failure in timber connections with multiple fasteners in a row. A prerequisite for use of the model is a known distribution on the fasteners of the total applied load. It is shown that the ideal plastic and linear elastic fracture mechanics solutions appear as special cases of the quasi-nonlinear model. The model offers strength predictions that include the effect of among others the number of fasteners in a row, fastener spacing, row spacing, end-distance, edge-distance, fastener diameter, and material properties, such as shear strength, fracture energy, and modulus of elasticity. Simple explicit expressions are obtained from the analysis for any known fastener load distribution. For bolted connections, where bolt-hole gaps cause high loads on certain bolts at random, the model may be used in simulation procedures or in probabilistic models.     

Effects of density profile of MDF on stiffness and strength of nailed joints

Nail-head pull-through, lateral nail resistance, and single shear nailed joint tests were conducted on medium density fiberboard (MDF) with different density profiles, and the relations between the results of these tests and the density profiles of MDF were investigated. The maximum load of nail-head pull-through and the maximum load of nailed joints were little affected by the density profile. However, the ultimate strength of lateral nail resistance, the stiffness, and the yield strength of nailed joints were affected by the density profile of MDF and showed high values when the surface layer of the MDF had high density. It is known that bending performance is also influenced by density profile. Therefore, the stiffness and the yield strength of nailed joints were compared with the bending performance of MDF. The stiffness of nailed joints was positively correlated with the modulus of elasticity (MOE); in the case of CN65 nails, the initial stiffness of joints changed little in response to changes in MOE. The yield strength of nailed joints had a high positive correlation with the modulus of rupture (MOR). The stiffness and the yield strength of nailed joints showed linear relationships with MOE and MOR, respectively.     

Analysis of strength grading of sawn timber based on numerical simulation

The main problem in machine strength grading of wood lies in the grade determining parameters and machine grading parameters, which are all stochastic, and the correlation between them is of critical importance. Therefore it is difficult to reliably determine the grade of a particular piece. In order to understand the effectiveness and accuracy of machine grading a large number of experiments should be performed in which the non-destructive machine grading parameters as well as destructive grade determining parameters should be determined. So far we have had access to only a limited number of such experiments. The basic idea of this paper is to use experimental data only for the determination of statistical parameters—mean vector and variance—covariance matrix—and afterwards use these statistical parameters in numerical simulation of machine grading. This numerical procedure gave some valuable data on the average values and the variability of grading results which now make it easier to decide what sample size should be used in determining machine settings.     

Estimation of stiffness and strength in timber knee joints with adhesive and verification by experiment

The use of adhesive joints is gradually increasing, especially those with glued-in steel rods (GIRs). There are, however, some problems with the design methods when used for moment-transmitting applications. In this article, design methods for GIRs and cross-lapped glued joints (CLJs) are proposed. For CLJs, we made a hypothesis that both rotational deformation of CLJ and stress of the glue line occurred with bending and shearing deformation of the timber. Using this hypothesis and Kelvin’s theorem, a mechanical model of CLJ is proposed. For GIRs, the axial stress component of the rod and the lateral stress component of the rod were taken into account using the theory of a beam on an elastic foundation. From the comparisons between calculations and experimental results, it was recognized that the stiffness and strength of CLJs and GIRs could be predicted precisely using our proposed models.     

Reliability analysis and duration-of-load strength adjustment factor of the rolling shear strength of cross laminated timber

In this study, the duration-of-load effect on the rolling shear strength of cross laminated timber (CLT), with different cross-sectional layups (five-layer and three-layer), was evaluated. A stress-based damage accumulation model is chosen to evaluate the duration-of-load strength adjustment factor of the rolling shear strength of CLT. This model incorporates the established short-term rolling shear strength of material and predicts the time to failure under arbitrary loading history. The model has been calibrated and verified based on the test data from low cycle trapezoidal fatigue tests (damage accumulation tests) in the previous study. The long-term rolling shear behaviour of CLT can then be evaluated from this verified model. As the developed damage accumulation model is a probabilistic model, it can be incorporated into a time based reliability assessment of the CLT products, considering short-term, snow, and dead load only loading cases. The reliability analysis results and factors reflecting the duration-of-load effect on the rolling shear strength of CLT are compared and discussed. The characteristic of this modeling theory lies in that the verified model is also able to predict the duration-of-load behaviour of CLT products under arbitrary loading history, such as long-term dead load case; then, these predictions of time to failure from the damage accumulation model can elucidate duration of load by the stress ratio evaluation approach. The results suggest that the duration-of-load rolling shear strength adjustment factor for CLT is more severe than the general duration-of-load adjustment factor for lumber; this difference should be considered in the introduction of CLT into the building codes for engineered wood design.     

Numerical simulation of embedding strength of glued laminated timber for dowel-type fasteners

A non-linear material model is proposed to describe the timber behavior. Anisotropic elasto-plastic constitutive law with hardening according to Hill yield criterion was used for the compressive behavior. Brittle behavior in tension and shear was modeled using the modified Hill failure criterion. The material model was implemented in a finite element code to simulate embedding strength of glued laminated timber for dowel-type fasteners. Reasonable agreement is found between numerical simulations and experimental measurements.     

Lateral performance of a semi-rigid timber frame structure: theoretical analysis and experimental study

Semi-rigidness of the joint connections is one of the main characteristics of timber structures. The pin-joint assumption for the semi-rigid joint connections might be not conservative in the timber structural design. In this paper, structural analysis was conducted on a semi-rigid timber portal frame; the formulas were derived in terms of the internal force and the lateral stiffness, and the influence of the semi-rigid connections was discussed. Moreover, experimental tests were performed on three full-scale timber portal frames and five bolted timber connections to study the lateral performance of the frames and the moment resistance of the connections. For consistency, the connections from the portal frames and the connections for bending tests were of the same configuration. Finally, a calculation flowchart of the lateral performance on a semi-rigid frame was presented to verify the derived formulas and to show a framework of the lateral structural design process.     

Improving strength grading of timber by grain angle measurement and mechanical modeling

Abstract

Timber strength grading has become a major issue in the European Union during the last years, due to the introduction of the Eurocode 5 and all its related standards. Currently, the most performing strength grading machines are able to locally detect the boards’ knots sizes and positions and interpret this information through adapted grading models. The best lead to improve their accuracy seems to be the introduction of new information about the boards and adapt the mechanical model to take them in account. Small grain angle causes high reduction of clear wood's mechanical properties; local value of slope of grain appears to be of high interest. The aim of this study is to quantify the additional accuracy that grain angle information can bring to an optical scanner used as a strength grading machine. A specific grading model has been developed accordingly, and the results obtained for different machine/model/loading combinations are presented. These results show that slope of grain measurement can significantly improve the accuracy of the optical scanner, for both modulus of elasticity and modulus of rupture estimations.